Electric motor

ABSTRACT

The invention relates to an electric motor, in particular for high speed operation, comprising a motor housing ( 4 ); inside the housing ( 4 ), a stator ( 2 ) having a core ( 8 ) and windings ( 9 ) toroidally wound on the core ( 8 ); and a rotor ( 1 ) inside the stator ( 2 ). The motor comprises an inductance controlling ring ( 3 ) in soft-magnetic material around the stator core ( 8 ), increasing for each phase the leakage inductance and forming for each phase an integrated filter inducatnce (L 1 , L 2 , L 3 ) outside the core ( 8 ) of the stator ( 2 ) but still inside the motor housing ( 4 ).

[0001] The present invention relates to an electric motor, in particularfor high speed operation, comprising a housing; inside this housing, astator having a core and windings toroidally wound on the core; andinside the stator, a rotor.

[0002] In high speed motors the distribution of losses differsconsiderably from the distribution of losses in standard speed motors.

[0003] The speed of these high speed motors is commonly frequencycontrolled by an inverter. Due to harmonics coming from this inverter,the eddy current losses can easily become the dominant loss.

[0004] As these eddy current losses increase as the square of the designspeed for the same frame size of the motor, these losses can be highcompared with standard speed motors.

[0005] Moreover, as these losses have a tendency to be concentrated tothe rotor, and cooling the rotor is most difficult, it is important tokeep these eddy current losses as small as possible.

[0006] One known way to reduce these losses is to reduce the harmoniccontent from the inverter. This can be done by adding a filter betweenthe inverter and the motor. In that way, the voltage supplied to themotor will have more the form of a sine wave.

[0007] If the eddy current losses are not magnitudes over what can betolerated in the motor, said filter could be separate inductors outsidethe motor serially connected with said windings of said stator; thoseinductors need to be cooled, usually by means of convection cooling.

[0008] High speed motors are normally of very high power density andtherefore often liquid cooled.

[0009] Separate standard convection cooled inductors outside the motortend to be big and heavy for high power motors. They also addconsiderable cost to the drive system.

[0010] In a motor with toroidal windings, only the winding half-turnslocated at the inside of the core, facing the rotor, contribute to thegeneration of the main magnetic field and therefore to the generation oftorque.

[0011] The stator winding half-turns at the outer side of said coregenerate a leakage flux and therefore form a part of the leakageinductance of the motor. This leakage inductance is not sufficient toreduce the harmonics due to inverter operation.

[0012] The invention seeks to provide a high speed electric motor whichdoes not have the above mentioned problem and which is suitable forinverter operation.

[0013] Therefore, an electric motor according to the invention,comprises an inductance controlling ring in soft-magnetic materialaround the stator core, increasing for each phase the leakage inductanceof the motor and forming an integrated filter inductance outside thecore of the stator but still inside the motor housing.

[0014] The ring avoids leakage of the magnetic flux towards the outside,for instance towards the outer housing of the motor. Leakage flux maycause considerable eddy current loss in a steel housing.

[0015] Between the stator core and the inductance controlling ring,there is a magnetic air gap. The thickness of this air gap determinesthe value of the integrated filter inductances formed by said ring, thestator core and the stator winding half-turns at the outside of thecore. This air gap is carefully calculated when designing the motor.

[0016] The motor will be somewhat larger, but the integrated filterinductances can use the same cooling system as the rest of the motor.There is no need for a separate cooling system which would take muchspace and add cost.

[0017] To limit eddy current losses in the inductance controlling ring,the resistance of this ring in the axial direction of the stator shouldbe high and therefore said ring is preferably laminated or from sinteredmaterial.

[0018] The motor may be liquid cooled with for example oil, in whichcase the stator may be housed in an internal housing in non magneticmaterial, for instance plastics, the inductance controlling ringsurrounding this internal housing.

[0019] Preferably the motor has an outer housing, in which case theinductance controlling ring is preferably supported either directly orby the intermediary of supports by this outer housing.

[0020] The invention will now be described by way of example and withreference to the accompanying drawings, in which:

[0021]FIG. 1 represents a longitudinal section of a high speed electricmotor, according to the invention;

[0022]FIG. 2 represents a cross section of the motor of FIG. 1, with theouter housing partly cut away;

[0023]FIG. 3 schematically shows the simplified electric circuit for thethree phase wound motor of FIGS. 1 and 2.

[0024] The motor shown in FIG. 1 is a three-phase permanent magnetsynchronous A.C electric motor, comprising essentially a rotor 1, astator 2 surrounding the rotor 1, an inductance controlling ring 3surrounding the stator 2, and an outer housing 4.

[0025] The rotor 1 consists of a permanent magnet part 5 mounted on anaxis 6, the end of which is supported in bearings 7 in the outer housing4.

[0026] The stator 2 has a tubular core 8 and is provided with a numberof stator windings 9 extending like rings around the core wall, withhalf-turns at the outside of the core 8 and half-turns at the inside ofthe core 8. The half-turns of the windings 9 are sunken in slots 10 inthe core 8, although in another embodiment the windings 9 are not inslots.

[0027] For containing cooling oil, the stator 2 is also surrounded by aninternal housing 11 in non-magnetic material, for example plastics.

[0028] The inductance controlling ring 3 is situated inside the motor,between the outer housing 4 and this internal housing 11.

[0029] This inductance controlling ring 3 is made from laminated steelor from sintered magnetic material.

[0030] The ring 3 is supported in the outer housing 4 by means ofsupports 12.

[0031] In some embodiments the supports 12 are omitted and theinductance controlling ring 3 is directly supported by the outer housing4, as its outer surface contacts the inner surface of the outer housing4.

[0032] In another embodiment the inductance controlling ring 3 is notsituated between the internal housing 11′ and the outer housing 4 butinside this internal housing 11.

[0033] The stator 2 may also be cooled by cooling air. In suchembodiment, the inner housing 11 may be omitted.

[0034] In any way, a magnetic air gap 13 is formed between the core 8 ofthe stator 2 and the inductance controlling ring 3, the width of thisgap 13 determining the value of the integrated filter inductance L1, L2or L3 formed for each of the phases by portions of the ring 3 outsidethe core 8 and the half-turns of the stator windings 9 for a phase,situated at the outside the core 8.

[0035] As shown in FIG. 3, each phase contains a number of windings 9forming inherently conventional internal inductances La, Lb and Lc.

[0036] These conventional internal inductances La, Lb and Lc depend notonly on the windings 9, but also on the geometry and the materialcharacteristics of the material on the inside of the stator core 8, i.e.rotor characteristics and airgap 13.

[0037] Each conventional internal inductance La, Lb and Lc is in serieswith an integrated filter inductance L1, L2 or L3 formed by the ring 3around the stator core 8, the stator core 8 itself and the windinghalf-turns for the phase situated at the outside of the core 8. Theseintegrated filter inductances L1, L2 and L3 depend on the gap 13, thegeometry and material characteristics on the outside of the stator core8, and the saturation level of the stator core. The saturation level ofsaid core 8 is normally kept low in order to minimise iron losses.

[0038] When the thickness of the air gap 13 decreases, the integratedfilter inductances L1, L2 and L3 increase. Consequently, the air gap 13is adjusted to the best compromise between a high integrated filterinductance for efficient reduction of inverter harmonics and additionaliron losses in the inductance controlling ring 3.

[0039] When the motor is operated, there will be some iron losses andsome eddy current losses. Compared to a standard high speed electricmotor, the motor according to the invention will have some added ironlosses, but substantially less eddy current losses, resulting in lesstotal losses. In particular the rotor losses will be reduced.

[0040] The inductance controlling ring 3 may in an alternativeembodiment be axially segmented.

[0041] In another alternative embodiment, the inductance controllingring 3 coincides with the motor housing 4. The motor housing 4 is thenmade from soft-magnetic material, contrary to known motor housings.

[0042] The present invention is not limited to the embodiments describedabove and represented in the figures, but such high speed electric motorcan have different embodiments and sizes, without leaving the scope ofthe invention as determined in the annexed claims.

1-12. (Canceled).
 13. Electric motor, in particular for high speedoperation, comprising a motor housing; inside this housing, a statorhaving a core and windings toroidally wound on the core, with half-turnsat the inside of the core and half-turns at the outside of the core; anda rotor inside the stator, said motor further including an inductancecontrolling ring in soft-magnetic material around the stator core,increasing for each phase the leakage inductance and forming for eachphase an integrated filter inductance (L1, L2, L3) outside the core ofthe stator but still inside the motor housing; and a magnetic air gapbetween the core of the stator and the inductance controlling ring, thethickness of this air gap determining the value of the inductances (L1,L2, L3) formed by said ring, the stator core and the stator windinghalf-turns outside the core.
 14. Electric motor according to claim 13wherein, in order to limit eddy current losses in the inductancecontrolling ring, the resistance of said ring in the axial direction ofthe stator is high.
 15. Electric motor according to claim 14, whereinsaid ring is laminated or made from sintered material.
 16. Electricmotor according to claim 13, wherein the motor is liquid cooled and thestator is housed in an internal housing in non-magnetic material, forinstance plastics, and wherein the inductance controlling ring surroundssaid internal housing.
 17. Electric motor according to claim 16, whereinthe motor is liquid cooled, and the inductance controlling ring isplaced inside said internal housing.
 18. Electric motor according toclaim 13, wherein the inductance controlling ring is supported eitherdirectly or by the intermediary of supports by the outer housing. 19.Electric motor according to claim 13, wherein the inductance controllingring coincides with the housing.
 20. Electric motor according to claim19, wherein the inductance controlling ring is axially segmented. 21.Electric motor according to claim 13, wherein the windings of the statorare ring-like around the wall of the core.
 22. Electric motor accordingto claim 21, wherein the windings of the stator are mounted in slots inthe core.
 23. Electric motor according to claim 13, wherein the motor isa permanent magnet synchronous AC motor.